Semiconductor memory

ABSTRACT

A semiconductor memory having a plurality of static random access memory cells, word lines, first and second bit lines orthogonal to the word lines, and threshold voltage control lines parallel to the word lines and each of the static random access memory cell includes the first and the second driver transistors, the first and the second load transistors, and the first and the second transfer transistors configured by Fin field effect transistors, and at least one of the Fin field effect transistors is configured by a separated-gate type double-gate field effect transistor comprising a first gate electrode and a second gate electrode and controlling a voltage for the first gate electrode to form a channel, and controlling a voltage for the second gate electrode to decrease a threshold voltage at the time of writing data.

CROSS REFERENCE TO RELATED APPLICATIONS AND INCORPORATION BY REFERENCE

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application P2006-016882 filed on Jan. 25, 2006;the entire contents of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor memory. In particular,it relates to the semiconductor memory having a plurality of StaticRandom Access Memory (SRAM) cells configured by Fin Field EffectTransistors (Fin FETs).

2. Description of the Related Art

Recently, as for the semiconductor device such as LSI, high performancehas been achieved by the miniaturization of the used device. In thescaling of the device, the gate length is reduced on the basis of aso-called scaling law, and the gate insulator is made thin film, in aMetal-Oxide Semiconductor Field-Effect Transistor (MOSFET) used for alogic circuit in a semiconductor device and a memory unit of StaticRandom Access Memory (SRAM) etc.

And then, a Fin FET that is Fully Depleted MOSFET of double-gate type isproposed in order to improve the cutoff characteristic that decreases bythe short-channel effect occurred by the transistor whose gate length Lis less than 30 nm (e.g., see Japanese patent Laid Open Publication No.H02-263473).

The Fin FET is a kind of three dimensional MIS type semiconductordevice, and the channel can be formed to the side view of the fin on twosurfaces by forming projection fin (Fin) that thinly excises silicon(Si) layer like the strip of paper, and making this fin overpass by thegate electrode. As for this Fin FET, the channel region of the fin FETis completely made depleted. Therefore, when a general-purposePoly-Silicon layer was used for the gate electrode, it was difficult toset the threshold voltage to the low threshold voltage (for example,0.2V or less in the absolute value) in which it aimed at the highcurrent drive.

When it tries to configure the circuit of Static Random Access MemoryCell (SRAM Cell) by using such the Fin FET, the threshold voltage of theFin FET cannot be properly controlled and the channel width cannot beset arbitrarily. Therefore, there was a problem with a difficult settingof the current transfer ratio of each Fin FET to a proper value. As aresult, since it was difficult for the SRAM Cell to obtain enough aStatic Noise Margin (SNM), the operational biasing point might beunstable, and it might be weak also to soft-error, etc., (e.g., see, E.J. Nowak, et al., “A Functional Fin FET-DGCMOS SRAM Cell”, InternationalElectron Devices Meeting (IEDM), Tech. Dig., IEEE, 2002, p. 411-414).

Moreover, the control of the potential of the channel region isperformed in order to obtain the low threshold voltage in which it aimsat the high current drive by the Fin FET (e.g., see, Y. X. Liu, et al,“Flexible Threshold Voltage Fin FETs with Independent Double-gates andan Ideal Rectangular Cross-Section Si-Fin Channel”, InternationalElectron Devices Meeting (IEDM), Tech. Dig., IEEE, 2003, p. 986-989).

The Fin FET in the above paper is called as a back-gate type MOSFET.Since the additional electrode layer for the control of the potential ofthe channel region was newly needed, the SRAM cell with the layout thatembedded the back-gate type Fin FET was not created.

SUMMARY OF THE INVENTION

An aspect of the present invention inheres in a semiconductor memoryhaving a plurality of static random access memory cells, a plurality ofword lines, and a plurality of first and second bit lines substantiallyorthogonal to the word lines. Each of the static random access memorycell includes a first inverter having a first driver transistor and afirst load transistor connected in series between a power supply voltageline and a ground line; a second inverter having a second drivertransistor and a second load transistor connected in series between thepower supply voltage line and a ground line; a first transfer transistorconnected in series between a first bit line and an output of the firstinverter; and a second transfer transistor connected in series between asecond bit line and an output of the second inverter, the output of thefirst inverter being connected to an input of the second inverter and aninput of the first inverter being connected to the output of the secondinverter. At least one of the first and the second driver transistors,the first and the second load transistors, and the first and the secondtransfer transistors is configured by a plurality of Fin field effecttransistor, and the Fin field effect transistor is configured by aseparated-gate type double-gate field effect transistor comprising afirst gate electrode and a second gate electrode, controlling a voltagefor the first gate electrode to form a channel, and controlling avoltage for the second gate electrode to decrease a threshold voltage atthe time of writing data.

Another aspect of the present invention inheres in a semiconductormemory having a plurality of static random access memory cells, aplurality of word lines, and a plurality of first and second bit linessubstantially orthogonal to the word lines. Each of the static randomaccess memory cell includes a first inverter having a first drivertransistor and a first load transistor connected in series between apower supply voltage line and a ground line; a second inverter having asecond driver transistor and a second load transistor connected inseries between the power supply voltage line and a ground line; a firsttransfer transistor connected in series between a first bit line and anoutput of the first inverter; and a second transfer transistor connectedin series between a second bit line and an output of the secondinverter, the output of the first inverter being connected to an inputof the second inverter and an input of the first inverter beingconnected to the output of the second inverter. The first and the seconddriver transistors, the first and the second load transistors, and thefirst and the second transfer transistors are configured by a pluralityof Fin field effect transistors, and both the first transfer transistorand the second transfer transistor are configured by a separated-gatetype double-gate field effect transistor comprising a first gateelectrode and a second gate electrode, controlling a voltage for thefirst gate electrode to form a channel, and controlling a voltage forthe second gate electrode to decrease a threshold voltage at the time ofwriting data.

Another aspect of the present invention inheres in a semiconductormemory having a plurality of static random access memory cells, aplurality of word lines, and a plurality of first and second bit linessubstantially orthogonal to the word lines. Each of the static randomaccess memory cell includes a first inverter having a first drivertransistor and a first load transistor connected in series between apower supply voltage line and a ground line; a second inverter having asecond driver transistor and a second load transistor connected inseries between the power supply voltage line and a ground line; a firsttransfer transistor connected in series between a first bit line and anoutput of the first inverter; and a second transfer transistor connectedin series between a second bit line and an output of the secondinverter, the output of the first inverter being connected to an inputof the second inverter and an input of the first inverter beingconnected to the output of the second inverter. The first and the seconddriver transistors, the first and the second load transistors, and thefirst and the second transfer transistors are configured by a pluralityof Fin field effect transistors, and both the first driver transistorand the second driver transistor are configured by a separated-gate typedouble-gate field effect transistor comprising a first gate electrodeand a second gate electrode, controlling a voltage for the first gateelectrode to form a channel, and controlling a voltage for the secondgate electrode to decrease a threshold voltage at the time of writingdata.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a semiconductor memory according toa first embodiment of the present invention.

FIG. 2 is an input-output characteristic of an inverter of thesemiconductor memory according to the first embodiment of the presentinvention.

FIG. 3 is a timing chart of a signal line of the semiconductor memoryaccording to the first embodiment of the present invention.

FIG. 4 is a plan view of the semiconductor memory according to the firstembodiment of the present invention.

FIG. 5 is a cross-sectional view taken in the line V-V of FIG. 4.

FIG. 6 is a cross-sectional view taken in the line V-V of FIG. 4 in thefabrication process of the semiconductor memory (No. 1) according to thefirst embodiment of the present invention.

FIG. 7 is a cross-sectional view taken in the line V-V of FIG. 4 in thefabrication process of the semiconductor memory (No. 2) according to thefirst embodiment of the present invention.

FIG. 8 is a cross-sectional view taken in the line V-V of FIG. 4 in thefabrication process of the semiconductor memory (No. 3) according to thefirst embodiment of the present invention.

FIG. 9 is a cross-sectional view taken in the line V-V of FIG. 4 in thefabrication process of the semiconductor memory (No. 4) according to thefirst embodiment of the present invention.

FIG. 10 is a cross-sectional view taken in the line V-V of FIG. 4 in thefabrication process of the semiconductor memory (No. 5) according to thefirst embodiment of the present invention.

FIG. 11 is a cross-sectional view taken in the line V-V of FIG. 4 in thefabrication process of the semiconductor memory (No. 6) according to thefirst embodiment of the present invention.

FIG. 12 is a cross-sectional view taken in the line V-V of FIG. 4 in thefabrication process of the semiconductor memory (No. 7) according to thefirst embodiment of the present

FIG. 13 is a cross-sectional view taken in the line V-V of FIG. 4 in thefabrication process of the semiconductor memory (No. 8) according to thefirst embodiment of the present invention.

FIG. 14 is a cross-sectional view taken in the line V-V of FIG. 4 in thefabrication process of the semiconductor memory (No. 9) according to thefirst embodiment of the present invention.

FIG. 15 is a cross-sectional view taken in the line V-V of FIG. 4 in thefabrication process of the semiconductor memory (No. 10) according tothe first embodiment of the present invention.

FIG. 16 is a cross-sectional view taken in the line V-V of FIG. 4 in thefabrication process of the semiconductor memory (No. 11) according tothe first embodiment of the present invention.

FIG. 17 is a cross-sectional view taken in the line V-V of FIG. 4 in thefabrication process of the semiconductor memory (No. 12) according tothe first embodiment of the present invention.

FIG. 18 is a cross-sectional view taken in the line V-V of FIG. 4 in thefabrication process of the semiconductor memory (No. 13) according tothe first embodiment of the present invention.

FIG. 19 is a plan view pattern in the fabrication process of thesemiconductor memory (No. 1) according to the first embodiment of thepresent invention.

FIG. 20 is a cross-sectional view taken in the line XX-XX of FIG. 19 inthe fabrication process of the semiconductor memory (No. 1) according tothe first embodiment of the present invention.

FIG. 21 is a cross-sectional view taken in the line XX-XX of FIG. 19 inthe fabrication process of the semiconductor memory (No. 2) according tothe first embodiment of the present invention.

FIG. 22 is a plan view pattern in the fabrication process of thesemiconductor memory (No. 2) according to the first embodiment of thepresent invention.

FIG. 23 is a cross-sectional view taken in the line XXIII-XXIII of FIG.22 in the fabrication process of the semiconductor memory (No. 1)according to the first embodiment of the present invention.

FIG. 24 is a cross-sectional view taken in the line XXIII-XXIII of FIG.22 in the fabrication process of the semiconductor memory (No. 2)according to the first embodiment of the present invention.

FIG. 25 is a cross-sectional view taken in the line XXIII-XXIII of FIG.22 in the fabrication process of the semiconductor memory (No. 3)according to the first embodiment of the present invention.

FIG. 26 is a cross-sectional view taken in the line XXIII-XXIII of FIG.22 in the fabrication process of the semiconductor memory (No. 4)according to the first embodiment of the present invention.

FIG. 27 is a plan view pattern in the fabrication process of thesemiconductor memory (No. 3) according to the first embodiment of thepresent invention.

FIG. 28 is a cross-sectional view taken in the line XXVIII-XXVIII ofFIG. 27 in the fabrication process of the semiconductor memory (No. 1)according to the first embodiment of the present invention.

FIG. 29 is a cross-sectional view taken in the line XXVIII-XXVIII ofFIG. 27 in the fabrication process of the semiconductor memory (No. 2)according to the first embodiment of the present invention.

FIG. 30 is a cross-sectional view taken in the line XXVIII-XXVIII ofFIG. 27 in the fabrication process of the semiconductor memory (No. 3)according to the first embodiment of the present invention.

FIG. 31 is a circuit diagram of a semiconductor memory according to asecond embodiment of the present invention.

FIG. 32 is a timing chart of a signal line of the semiconductor memoryaccording to the second embodiment of the present invention.

FIG. 33 is a plan view pattern of the semiconductor memory according tothe second embodiment of the present invention.

FIG. 34 is a cross-sectional view taken in the line XXXIV-XXXIV of FIG.33.

FIG. 35 is a plan view pattern in the fabrication process of thesemiconductor memory (No. 1) according to the second embodiment of thepresent invention.

FIG. 36 is a plan view pattern in the fabrication process of thesemiconductor memory (No. 2) according to the second embodiment of thepresent invention.

FIG. 37 is a plan view pattern in the fabrication process of thesemiconductor memory (No. 3) according to the second embodiment of thepresent invention.

FIG. 38 is a circuit diagram of a semiconductor memory according to athird embodiment of the present invention.

FIG. 39 is a plan view of the semiconductor memory according to thethird embodiment of the present invention.

FIG. 40 is a cross-sectional view taken in the line XL-XL of FIG. 39.

DETAILED DESCRIPTION OF THE INVENTION

Referencing the drawings, the first to the third embodiment according tothe present invention are explained forthwith. The same or similarsymbols are applied to the same or similar parts throughout the appendeddrawings. However, it should be noted that the drawings are merelyschematics and that the relationship between thickness and planardimension, and the ratio of respective layer thicknesses and the likediffer from those of the actual invention. Accordingly, specificthicknesses and dimensions should be determined while considering thefollowing description. Furthermore, needless to say that parts withdiffering dimensions and/or differing ratios among the drawings may beincluded.

In addition, the first to the third embodiments given forthwithillustrate devices and methods for embodying the technical idea of thepresent invention, and that technical idea of the present invention isnot limited to the following materials, shapes, structures, arrangementsor the like. The technical idea of the present invention may be modifiedinto various modifications within the scope of the appended claims.

According to the semiconductor memory of the embodiments of the presentinvention, the semiconductor memory including the SRAM Cell having aperformance of enough amount of SNM can be obtained, while the back-gatetype Fin FETs are used for the transistors providing with the SRAM Cell.

First Embodiment

A semiconductor memory according to the first embodiment of the presentinvention includes a static random access memory (SRAM) Cell as shown inFIG. 1. The SRAM Cell includes six transistors TR1 to TR6.

The transfer transistor TR5 is connected with a bit line BLT, is ann-channel FET, and is called ‘transfer transistor’ or ‘path-gatetransistor’.

Moreover, the transfer transistor TR5 is a double-gate FET configured bya Fin FET, and has a gate electrode G1 in one side of the sidewallsurface that the Fin opposes and a gate electrode G2 in the othersidewall surface. In addition, two gate electrodes G1 and G2 aredescribed to show that the transistors TR1 to TR6 including the transfertransistor TR5 are the double-gate FETs, as shown in FIG. 1.

The transfer transistor TR5 is separated-gate type FET, the gateelectrode G1 is connected with a word line WL, and the gate electrode G2is connected with a threshold voltage control line VtC. The drain of thetransfer transistor TR5 is connected with the bit line BLT, and thesource of the transfer transistor TR5 is connected with a node Vout1.

The transfer transistor TR6 is configured by a n-channel FET, the drainof which is connected with a bit line BLC, and called ‘transfertransistor’ or ‘path-gate transistor’. Moreover, the transfer transistorTR6 is a double-gate FET configured by a Fin FET, and has a gateelectrode G1 in one side of the sidewall surface that the Fin opposesand a gate electrode G2 in the other sidewall surface.

The transfer transistor TR6 is configured by separated-gate type FET,the gate electrode G1 is connected with a word line WL, and the gateelectrode G2 is connected with a threshold voltage control line VtC. Thedrain of the transfer transistor TR6 is connected with the bit line BLC,and the source of the transfer transistor TR5 is connected with a nodeVout2.

The driver transistor TR3 is configured by a n-channel FET, and called‘driver transistor’ or ‘pull-down transistor’. Moreover, the drivertransistor TR3 is a double-gate FET configured by a Fin FET, and has agate electrode G1 in one side of the sidewall surface that the Finopposes and a gate electrode G2 in the other sidewall surface. The gateelectrode G1 and the gate electrode G2 of the driver transistor TR3 areconnected mutually and together, and two gate electrodes G1 and G2 ofthe driver transistor TR3 are connected with a node Vin1. The drain ofthe driver transistor TR3 is connected with the node Vout1, and thesource of the driver transistor TR3 is connected with a ground line VSS1at the ground level.

The driver transistor TR4 is configured by n-channel FET, and calleddriver transistor or pull-down transistor. Moreover, the drivertransistor TR4 is double-gate FET configured by Fin FET, and has a gateelectrode G1 in one side of the side surface that the Fin opposes and agate electrode G2 in the other side surface.

The gate electrode G1 and the gate electrode G2 of the driver transistorTR4 are connected mutually and together, and two gate electrodes G1 andG2 of the driver transistor TR4 are connected with a node Vin2. Thedrain of the driver transistor TR4 is connected with the node Vout2, andthe source of the driver transistor TR4 is connected with a ground lineVSS2 at the ground level.

The load transistor TR1 is configured by p-channel FET, and called loadtransistor or pull-up transistor.

Moreover, the load transistor TR1 is double-gate FET configured by FinFET, and has a gate electrode G1 in one side of the side surface thatthe Fin opposes and a gate electrode G2 in the other side surface. Thegate electrode G1 and the gate electrode G2 of the load transistor TR1are connected mutually and together, and two gate electrodes G1 and G2of the load transistor TR1 are connected with the node Vin1. The drainof the load transistor TR1 is connected with a power supply voltage VDD,and the source of the load transistor TR1 is connected with the nodeVout1.

The load transistor TR2 is configured by p-channel FET, and called loadtransistor or pull-up transistor.

Moreover, the load transistor TR2 is double-gate FET configured by FinFET, and has a gate electrode G1 in one side of the side surface thatthe Fin opposes and a gate electrode G2 in the other side surface.

The Gate electrode G1 and the gate electrode G2 of the load transistorTR2 are connected mutually and together, and two gate electrodes G1 andG2 of the load transistor TR2 are connected with the node Vin2. Thedrain of the load transistor TR2 is connected with a power supplyvoltage VDD, and the source of the load transistor TR1 is connected withthe node Vout2.

The stability of the SRAM Cell is determined by the beta ratio of thecurrent drive of a driver transistor to the current drive of a transfertransistor. The stability factor of the SRAM Cell can be increased byincreasing the value of the beta ratio to determine the value of thecurrent drive of the driver transistor being in large or more than thevalue of the current drive of the transfer transistor. In order todetermine the value of beta ratio of the current drive in large and toimprove the stability of the SRAM Cell, the channel width of the drivertransistor can be performed by setting in large and controlling thevalue of the threshold voltage Vt at a proper value, in the case of theFETs other than the Fin FETs.

However, in the six transistors/cell type SRAM Cell as shown in FIG. 1,the difficulty occurs to improve the stability of the SRAM Cell bytrying to configure each transistor by the Fin FET as mentioned below.

(1) It is difficult to adjust the value of the current drive of then-channel Fin FETs which configure the driver transistor TR3 and TR4 andthe transfer transistors TR5 and TR6 by adjusting the value of thechannel width, while the value of the current drive of the conventionaln-channel FETs can be adjusted by optimizing the channel width of theFETS.

The channel width of the Fin FETs is determined by the height of thesilicon projection or protrusion that is called Fin. Therefore, it issubstantially difficult to change the height of the Fin with each of theFin FETs. If it tries to change the height of the Fin with each of theFin FETs, it is necessary to perform processing of lithography andReactive Ion Etching (RIE), etc, by different processing steps in eachof the Fin FETs. Therefore, it is considered that the efficiency of thefabrication processing is very bad.

(2) A method for adjusting the value of the gate length of each of theFin FETs can be considered in order to adjust the value of the currentdrive in each of the Fin FETs. However, it is difficult to obtain theenough value of beta ratio of the current drive for the method ofadjusting the value of the gate length of each of the Fin FETs.Moreover, since there is a plurality of Fin FETs with different gatelength in the SRAM Cell, the Critical Dimension (CD) control of thelithography processing becomes difficult. Moreover, since the size ofthe fine line pattern that exceeds the limitation of the lithography bylight etc, is formed, if the gate length of the Fin FETs in the SRAMCell is not a single gate length, it is difficult for performing thesidewall transfer process for the gate electrode G1 and G2 of the FinFETs.

(3) Furthermore, in the Fin FETs, even if the metal gate electrode madeof the conductor with a work function near the mid gap energy level ofthe Fin semiconductor can be used, the span of adjustable range of thethreshold voltage of the Fin FETs is a comparatively small. Therefore,it is difficult to obtain a high level of the threshold voltage Vt, forinstance, +0.3V or more enough necessary for cut-off of the conductingcurrent of the Fin FETs.

Therefore, it proposes using the back-gate type, so-called theseparated-gate type Fin FETs, as show in FIG. 1, as a method forconfiguring the SRAM Cell by using the Fin FETs in the first embodiment.

In FIG. 1, the threshold voltage control line (Vt Control line) VtC isconnected with the gate electrodes G2 that are the back-gate of thetransfer transistors TR5 and TR6, respectively. The threshold voltagecontrol line VtC controls the value of the threshold voltage of thetransfer transistors TR5 and TR6 by applying the value of negativevoltage on the gate electrodes G2 comparing to the value of the groundpower supply voltages VSS1 and VSS2.

According to such the threshold voltage control, the transfertransistors TR5 and TR6 are designed to have the value of low thresholdvoltage in order to achieve the large amount of current conductingthrough the transfer transistors TR5 and TR6, at the time ofwriting/reading mode when the applied voltage for the word line WL is atthe high level.

On the other hand, the transfer transistors TR5 and TR6 can be designedto have the value of comparatively high threshold voltage in order toreduce the amount of leakage current conducting through the transfertransistors TR5 and TR6, at the time of except writing/reading mode whenthe applied voltage for the word wine WL becomes at low level and data Dis stored at the memory cell.

The semiconductor memory according to the first embodiment includes theSRAM Cell, and the SRAM Cell is configured by a plurality of Fin FETs(TR1 through TR6). Each at least one of the Fin FETs (TR5 and/or TR6)includes the first gate electrode G1 and the second gate electrode G2,respectively. The applied voltage for the first gate electrode G1 of theFin FET (TR5 and/or TR6) is controlled, and then the channel is formedin the Fin FET (TR5 and/or TR6). The applied voltage for the second gateelectrode G2 of the Fin FET (TR5 and/or TR6) is controlled, and then theapplied voltage for the channel is controlled in the Fin FET (TR5 and/orTR6). As a result, the value of threshold voltage of the Fin FET (TR5and/or TR6) can be reduced at the time of writing data mode.

Therefore, the transfer transistors TR5 and TR6 are configured by thedouble-gate Fin FETs having the separated-gate type structure. Differentvoltages are applied to the first gate electrode G1 and the second gateelectrode G2, respectively, in the transfer transistors TR5 and TR6. Theoperational mode of the double-gate type transistor and the operationalmode of the back-gate type transistor can be achieved by applying thedifferent voltage to the first gate electrode G1 and the second gateelectrode G2 in the transfer transistors TR5 and TR6.

And then, it is possible to obtain the Fin FETs (TR5 and TR6) with theproper threshold voltage and also it is possible to adjust the currentdrive of the Fin FETs, by configuring the SRAM Cell using the back-gatetype Fin MOSFETs (TR5 and TR6) having the separated-gate structure.

The first channel region is formed with the first gate electrode G1 ofthe transfer transistors TR5 and TR6, on the other hand, the appliedvoltage of the first channel region can be controlled by the voltageapplied at the second gate electrode G2 of the transfer transistors TR5and TR6. The second channel region is formed with the second gateelectrode G2 of the transfer transistors TR5 and TR6. The current driveof the transfer transistors TR5 and TR6 can be increased withconfiguring the second channel region in addition to the first channelregion. The cut-off characteristic of the transfer transistors TR5 andTR6 can be also improved with configuring the second channel region inaddition to the first channel region. It is also possible to improve theSNM by reducing the value of the threshold voltage at the time ofwriting mode and by increasing the value of the threshold voltage duringat the time of data storing mode, by using the back-gate type FinMOSFETs (TR5 and TR6) in the SRAM Cell.

Thus, the beta ratio of the current drive of the driver transistor tothe current drive of the transfer transistor can be changed by theseries of time variation.

As a result, the SNM can be increased as shown in FIG. 2. That is, ifthe electrical current is conducting with the transfer transistor TR6,the input voltage Vin1 rises steep like being high output voltage Vout1in input-output characteristics 31 of the input voltage Vin1 and theoutput voltage Vout1 of the first inverter that configures a flip-flopF/F of the SRAM Cell. Therefore, since it becomes direction where thecurve 31 in the upper part of loop 33 of upper left of the butterflycurve obtains more margins, the SNM can be increased.

If the electrical current is conducting with the transfer transistorTR5, the input-output characteristics 32 of the input voltage Vin2 andthe output voltage Vout2 of the second inverter that configures theflip-flop F/F of the SRAM Cell becomes the horizontal steep. Therefore,the SNM can be increased since it becomes direction where the curve 32in the upper part of loop 34 of lower right of the butterfly curveobtains more margins.

As shown in FIG. 3, the signal voltage applied to the threshold voltagecontrol line VtC changes synchronizing with the write enable signal WRof the SRAM Cell. The second gate electrodes G2 of the transfertransistors TR5 and TR6 are connected with the threshold voltage controlline VtC. Therefore, the applied voltage for the second gate electrodesG2 of the transfer transistors TR5 and TR6 change synchronizing with thewrite enable signal WR of the SRAM Cell, and the value of the thresholdvoltage of transfer transistors TR5 and TR6 becomes lower synchronizingwith the write enable signal WR.

The device characteristic of the transfer transistors TR5 and TR6 can bechanged by changing the applied signal voltage to threshold voltagecontrol line VtC synchronizing with the write enable signal WR.

The signal voltage applied to the threshold voltage control line VtCprecedes the timing of writing/reading mode of the memory cell and isapplied at the time t21, and the threshold voltage Vt of the transfertransistors TR5 and TR6 is set at the time t21 earlier than thewriting/reading time.

More specifically, as shown in FIG. 3, the signal voltage of thethreshold voltage control line VtC is input with the margin and risesand falls at the time t21 before the time when write enable signal WR isturned on, and the transfer transistors TR5 and TR6 of the low thresholdvoltage are turned on in the low-threshold (low-Vt) mode and the data Dis written in the SRAM Cell.

The signal voltage of the threshold voltage control line VtC is designedto begin the rise with the margin at the time t22 after the start timeof turning-off when the write enable signal WR is turned off. When thesignal voltage of the threshold voltage control line VtC rises, thehigh-threshold (high-Vt) mode is set to the SRAM Cell, and the transfertransistors TR5 and TR6 having a high threshold voltage are turned offand the written data D is stored in the SRAM Cell.

In addition, if the time t22 when the signal voltage of the thresholdvoltage control line VtC rises is delayed to a large extent from thetiming when the write enable signal WR is turned off, the transfertransistors TR5 and TR6 are set to have a low threshold voltage evenwhen the transfer transistors TR5 and TR6 should be turned off.Therefore, it is preferable that the signal voltage of the thresholdvoltage control line VtC rises as early as possible from the timing whenthe write enable signal WR is turned off. On the other hand, thresholdvoltage Vt can be set to a low value when the signal voltage of thethreshold voltage control line VtC should rise earlier than the timingof falling of the write enable signal WR at the time of the rise of thesignal voltage of the threshold voltage control line VtC.

In the timing of the rise of the signal voltage of the threshold voltagecontrol line VtC, the trade-off like the above-mentioned exists.Therefore, the timing of the rise of the signal voltage of the thresholdvoltage control line VtC can be set to optimal timing.

As mentioned above, the applied voltage for the second gate electrodesG2 of the transfer transistors TR5 and TR6 that synchronize with thewrite enable signal WR changes at a time earlier than a time of the riseof the write enable signal WR. At the time, the transfer transistors TR5and TR6 operate in low threshold (low-Vt) mode. And also, the transfertransistors TR5 and TR6 are in the low threshold (low-Vt) mode whilehaving changed until a time later than a time of the falling of thewrite enable signal WR.

The signal voltage of the threshold voltage control line VtC rises laterthan the falling of the write enable signal WR. As a result, thethreshold voltage Vt is held in the low state value, and the cut-offcharacteristic of the transfer transistors TR5 and TR6 can be improvedafter time passes to some degree.

As shown in FIG. 4 and FIG. 5, in the semiconductor memory according tothe first embodiment, silicon (Si) Fins 3 a to 3 d are provided on asilicon oxide layer 2. The silicon Fin 3 a is used for the active areaof the driver transistor TR3 and the active area of the transfertransistor TR5. The silicon Fin 3 b is used for the active area of theload transistor TR1. The silicon Fin 3 c is used for the active area ofthe load transistor TR2. The silicon Fin 3 d is used for the active areaof the driver transistor TR4 and the active area of the transfertransistor TR6.

Cap layers 4 a to 4 d are provided on the silicon Fins 3 a to 3 d,respectively.

Gate electrodes 6 a to 6 j are provided on the silicon oxide layer 2.The silicon Fin 3 a and the cap layer 4 a have two side surfacesmutually opposed. The gate electrodes 6 a and 6 j configured byPoly-Silicon (poly-Si) contact to one side surface of the silicon Fin 3a and the cap layer 4 a. The gate electrode 6 a functions as the gateelectrode G2 of the driver transistor TR3 shown in FIG. 1. The gateelectrode 6 j functions as the gate electrode G2 of the transfertransistor TR5. The gate electrodes 6 b and 6 i configured by thePoly-Silicon contact to the other side surface of the silicon Fin 3 aand the cap layer 4 a. The gate electrode 6 b functions as the gateelectrode G1 of the driver transistor TR3 of FIG. 1 and the gateelectrode G1 of the load transistor TR1. The gate electrode 6 ifunctions as the gate electrode G1 of the transfer transistor TR5.

As the above-mentioned, the driver transistor TR3 is configured by theFin FET of which the Fin is silicon Fin 3 a, and is double-gate FETwhose gate electrodes are two gate electrodes 6 a and 6 b. The transfertransistor TR5 is configured by the Fin FET of which the Fin is siliconFin 3 a, and is double-gate FET whose gate electrodes are two gateelectrodes 6 i and 6 j.

The silicon Fin 3 b and the cap layer 4 b have two side surfacesmutually opposed. The gate electrode 6 b is contacted to one sidesurface of the silicon Fin 3 b and the cap layer 4 b. The gateelectrodes 6 c and 6 h configured by the Poly-Silicon contact to theother side surface of the silicon Fin 3 b and the cap layer 4 b. Thegate electrode 6 c functions as the gate electrode G2 of the loadtransistor TR1 shown in FIG. 1. The gate electrode 6 h functions as thegate electrode G2 of the load transistor TR2. The load transistor TR1 isconfigured by the Fin FET of which the Fin is silicon Fin 3 b, and isdouble-gate FET whose gate electrodes are two gate electrodes 6 b and 6c.

The silicon Fin 3 c and the cap layer 4 c have two side surfacesmutually opposed. The gate electrodes 6 c and 6 h configured by thePoly-Silicon contact to one side surface of the silicon Fin 3 c and thecap layer 4 c. The gate electrode 6 g configured by the Poly-Siliconcontacts to the other side surface of the silicon Fin 3 c and the caplayer 4 c. The gate electrode 6 g functions as the gate electrode G1 ofthe load transistor TR2 of FIG. 1 and the gate electrode G1 of thedriver transistor TR4. The load transistor TR2 is configured by the FinFET of which the Fin is silicon Fin 3 c, and is double-gate FET whosegate electrodes are two gate electrodes 6 g and 6 h.

The silicon Fin 3 d and the cap layer 4 d have two side surfacesmutually opposed. The gate electrodes 6 d and 6 g configured byPoly-Silicon contact to one side surface of the silicon Fin 3 d and thecap layer 4 d. The gate electrode 6 d functions as the gate electrode G1of the transfer transistor TR6 shown in FIG. 1. The gate electrodes 6 eand 6 f configured by the Poly-Silicon contact to the other side surfaceof the silicon Fin 3 d and the cap layer 4 d. The gate electrode 6 efunctions as the gate electrode G2 of the transfer transistor TR6 shownin FIG. 1. The gate electrode 6 f functions as the gate electrode G2 ofthe driver transistor TR4. The driver transistor TR4 is configured bythe Fin FET of which the Fin is silicon Fin 3 d, and is double-gate FETwhose gate electrodes are two gate electrodes 6 f and 6 g. The transfertransistor TR6 is configured by the Fin FET of which the Fin is siliconFin 3 d, and is double-gate FET whose gate electrodes are two gateelectrodes 6 d and 6 e.

An interlayer insulator 9 is configured by the silicon oxide film, andprovided on the silicon oxide layer 2, the cap layers 4 a to 4 d and thegate electrodes 6 a to 6 j. The interlayer insulator 9 contacts to thesilicon Fins 3 a to 3 d, the cap layers 4 a to 4 d, and the sidesurfaces of the gate electrodes 6 a to 6 j. The surface of theinterlayer insulator 9 is planarized.

Contact plugs 8 a to 8 j are provided on each silicon Fin 3 a to 3 d ateach position of the corresponding contact holes 8 a to 8 j so as topass through the interlayer insulator 9.

Contact plugs 12 a to 12 j are provided on each corresponding gateelectrode 6 a to 6 j at each position of the corresponding contact holes11 a to 11 j so as to pass through the interlayer insulator 9.

M1 electrode layers 13 a to 13 n are provided on the interlayerinsulator 9, the contact plugs 8 a to 8 j, and the contact plugs 12 a to12 j. The M1 electrode layer 13 a connects the gate electrodes 6 a to 6c. As a result, the gate electrodes G1 and G2 of the driver transistorTR3 are connected with the gate electrodes G1 and G2 of the loadtransistor TR1.

The M1 electrode layer 13 m connects the gate electrodes 6 f to 6 h. Asa result, the gate electrodes G1 and G2 of the load transistor TR2 areconnected with the gate electrodes G1 and G2 of the driver transistorTR4. On the other hand, neither gate electrodes 6 i nor 6 j areconnected by the M1 electrode layers. As a result, the gate electrodesG1 and G2 of the transfer transistor TR5 are not commonly connected, andthe transfer transistor TR5 is separated-gate type double-gate FET.Similarly, since neither gate electrodes 6 d nor 6 e are connected,neither the gate electrodes G1 nor G2 of the transfer transistor TR6 arecommonly connected. Therefore, the transfer transistor TR6 isseparated-gate type double-gate FET.

An interlayer insulator 14 is configured by the silicon oxide film, andprovided on the interlayer insulator 9 and the M1 electrode layers 13 ato 13 n. The interlayer insulator 14 is contacted to the side surface ofthe M1 electrode layers 13 a to 13 n. The surface of the interlayerinsulator 14 is planarized.

VIA1 plugs 16 a to 16 j are provided on each corresponding M1 electrodelayer 13 a to 13 n at each position of the corresponding VIA1 hole 15 aso as to pass through the interlayer insulator 14.

A word line WL and M2 electrode layers 17 b to 17 g, 17 i and 17 j areprovided on the interlayer insulator 14 and the VIA1 plugs 16 a to 16 j.The word line WL is connected with the gate electrode 6 d through theVIA1 plug 16 a, the M1 electrode layer 13 b, and the contact plug 12 d.As a result, the gate electrode G1 of the transfer transistor TR6 isconnected with the word line WL. Moreover, the word line WL is connectedwith the gate electrode 6 i through the VIA1 plug 16 h, the M1 electrodelayer 13 k, and the contact plug 12 i. As a result, the gate electrodeG1 of the transfer transistor TR5 is connected with the word line WL.

An interlayer insulator 18 is configured by the silicon oxide film, andprovided on the interlayer insulator 14, the word line WL, and the M2electrode layers 17 b to 17 g, side surface of the word line WL and M2electrode layers 17 b to and 17 g, 17 i and 17 j. The surface of theinterlayer insulator 18 is planarized.

VIA2 plugs 19 a to 19 h are provided on each corresponding word line WLand M2 electrode layers 17 b to 17 g, 17 i and 17 j at each position ofthe corresponding VIA2 holes so as to pass through the interlayerinsulator 18.

A threshold voltage control line VtC, bit lines BLT and BLC, groundlines VSS1 and VSS2 at the ground level, and a power supply voltage VDDare provided on the interlayer insulator 18 and the VIA2 plugs 19 a to19 h. The threshold voltage control line VtC is connected with the gateelectrode 6 j through the VIA2 plug 19 a, the M2 electrode layer 17 f,the VIA1 plug 16 f, the M1 electrode layer 13 i, and the contact plug 12j. As a result, the gate electrode G2 of the transfer transistor TR5 isconnected with the threshold voltage control line VtC. Moreover, thethreshold voltage control line VtC is connected with the gate electrode6 e through the VIA2 plug 19 h, the M2 electrode layer 17 e, the VIA1plug 16 e, the M1 electrode layer 13 c, and the contact plug 12 e. As aresult, the gate electrode G2 of the transfer transistor TR6 isconnected with the threshold voltage control lines VtC.

The bit line BLT is connected with the active area 3 a through the VIA2plug 19 b, the M2 electrode layer 17 g, the VIA1 plug 16 g, the M1electrode layer 13 j, and the contact plugs 8 c. As a result, the drainof the transistor TR5 is connected with the bit line BLT.

The bit line BLC is connected with the active area 3 d through the VIA2plug 19 g, the M2 electrode layer 17 d, the VIA1 plug 16 d, the M1electrode layer 13 f, and the contact plugs 8 f. As a result, the drainof the transfer transistor TR6 is connected with the bit line BLC.

The ground line VSS1 for the ground level is connected with the activearea 3 a through the VIA2 plug 19 e, the M2 electrode layer 17 b, theVIA1 plug 16 b, the M1 electrode layer 13 d, and the electric contactplug 8 e. As a result, the source of the driver transistor TR3 isconnected with the ground line VSS1 for the ground level.

The ground line VSS2 for the ground level is connected with the activearea 3 d through the VIA2 plug 19 d, the M2 electrode layer 17 j, theVIA1 plug 16 j, the M1 electrode layer 13 n, and the electric contactplug 8 h. As a result, the source of the driver transistor TR4 isconnected with the ground line VSS2 for the ground level.

The power supply voltage VDD is connected with the active area 3 bthrough the VIA2 plug 19 f, the M2 electrode layer 17 c, the VIA1 plug16 c, the M1 electrode layer 13 e, and the contact plug 8 j. As aresult, the drain of the load transistor TR1 is connected with the powersupply voltage VDD. Moreover, the power supply voltage VDD is connectedwith the active area 3 c through the VIA2 plug 19 c, the M2 electrodelayer 17 i, the VIA1 plug 16 i, the M1 electrode layer 13 l, and thecontact plug 8 i. As a result, the drain of the transistor TR2 isconnected with the power voltage VDD.

A passivation layer 20 is configured by the silicon oxide film, andprovided on the interlayer insulator 18, the threshold voltage controllines VtC, the bit lines BLT and BLC, the ground lines VSS1 and VSS2,and the power supply voltage VDD. The passivation layer 20 contacts tothe side surface of the threshold control lines VtC, the bit lines BLTand BLC, the ground lines VSS1 and VSS2 at the ground level, and thepower supply voltage VDD. The surface of the passivation layer 20 isplanarized.

In the SRAM Cell, all of the transistors TR1 to TR6 have the first gateelectrode G1 and the second gate electrode G2. As for the transistorsTR1 to TR4, the first gate electrode G1 and the second gate electrode G2are connected through the M1 electrode layer. As for the transfertransistors TR5 and TR6, the electrode layer is provided for a differentapplied voltage to be given to the first gate electrode G1 and thesecond gate electrode G2. In the SRAM Cell, the transfer transistors TR5and TR6 that has back-gate type, so-called separated-gate typestructure, and the transistors TR1 to TR4 that has common-gate typestructure are embedded.

Moreover, until the fabrication processing of the gate electrodes G1 andG2, the SRAM Cell can be configured with the double-gate type Fin FETsof the same structure. As a result, a plurality of gate electrodes ofthe same structure and a plurality of Fins of the same structure areonly formed during the course of the processing of a plurality of FinFETs. Therefore, enough process margins can be allowed for thelithography processing.

If the structures of the gate electrode and the Fin of the Fin FETS arevarious or the Fin FETs more than two varieties exist, the fabricationprocessing becomes complex. Moreover, the number of control parametersin the fabrication processing increases, and then the process marginsbecome small and the fabrication processing steps become difficult.

In the SRAM Cell, the threshold voltage control line VtC that providesthe voltage to the second gate electrode G2 is located orthogonal to theword line WL. The SRAM Cell according to the first embodiment can beconfigured while following the layout of the conventional type since thethreshold voltage control line VtC intersects with the word line WL. Ifthe threshold voltage control line VtC is located in parallel to theword line WL, it is necessary to increase the number of metal layers orit is necessary to bend the word line WL. Therefore, it isdisadvantageous in the processing yield of the semiconductor memory.

In the SRAM Cell, the threshold voltage control line VtC that providesthe voltage to the second gate electrode G2 of the transfer transistorsTR5 and TR6 is shared with an adjacent cell. That is, the semiconductormemory has a plurality of adjacent SRAM Cells mutually sharing thethreshold voltage control line VtC.

The threshold voltage control line VtC that provides the voltage appliedto the second gate electrode G2 of the transfer transistor TR5/TR6 of acertain SRAM Cell is shared with the threshold voltage control line VtCthat provides the voltage applied to the second gate electrode G2 of thetransfer transistor TR5/TR6 of an adjacent SRAM Cell in the directionelongating with the word line WL.

The minimization of the area can be achieved by sharing the thresholdvoltage control line VtC with an adjacent SRAM Cell. Although the VSSline was shared in the conventional type, two VSS lines are provided inthe cell in the layout of the first embodiment.

(Fabrication Method)

Next, the fabrication method for the semiconductor memory according tothe first embodiment of the present invention will be explained.

As shown in FIG. 6, the SOI substrate 1 of which the silicon (Si) layer3 is provided on the silicon oxide (SiO₂) layer 2 is prepared.

Next, as shown in FIG. 7, the cap layer 4 configured by silicon nitride(Si₃N₄) etc. is deposited on the semiconductor (silicon) layer 3 byusing the Chemical Vapor Deposition (CVD) method.

As shown in FIG. 8, the resist film is formed on the cap layer 4, theresist film is patterned by the lithography processing, and thepatterned resist films 5 a to 5 d are formed. The resist films 5 a to 5d are patterned according to the pattern of the silicon Fins describedlater.

As shown in FIG. 9, the cap layer 4 is etched by the Reactive IonEtching (RIE) method, by using the resist films 5 a to 5 d as a mask andthe silicon layer 3 as a stopper. As a result, cap layer 4 is patternedaccording to the pattern of the silicon Fin described later. And then,the patterned cap layers 4 a to 4 d are formed.

As shown in FIG. 10, the resist films 5 a to 5 d are removed.

And then, the silicon layer 3 is etched by the RIE method, by using thecap layers 4 a to 4 d as a mask and the silicon oxide layer 2 as astopper. As a result, the silicon Fins 3 a to 3 d configured by thesilicon layer 3 are formed. The silicon Fin 3 a is used for the activearea of the driver transistor TR3 and the transfer transistor TR5, thesilicon Fin 3 b is used for the active area of the load transistor TR1,the silicon Fin 3 c is used for the active area of the load transistorTR2, and the silicon Fin 3 d is used for the active area of the drivertransistor TR4 and the transfer transistor TR6. Therefore, the siliconFins 3 a to 3 d are doped if necessary. And then, the gate insulator isformed with the oxidation of the exposed surface of the silicon Fins 3 ato 3 d.

As shown in FIG. 11, the conducting film 6 configured by Poly-Silicon(Poly-Si) is deposited on the silicon oxide layer 2 by the CVD method.The conducting film 6 is also deposited in surroundings of the siliconFins 3 a to 3 d as contacting to the side surface of the silicon Fins 3a to 3 d. The silicon Fins 3 a to 3 d and cap layers 4 a to 4 d areembedded by the conducting film 6.

As shown in FIG. 12, polishing of the conducting film 6 is performed bythe Chemical Mechanical Polishing (CMP) method, by using the cap layers4 a to 4 d as a stopper.

As shown in FIG. 13, the resist film 7 is formed on the conducting film6 and the cap layers 4 a to 4 d, and the resist film 7 is patterned bythe lithography processing. The resist film 7 is patterned according tothe pattern of the gate electrode described later and the pattern of thecontact hole on the silicon Fin. The pattern of the gate electrode isprovided to both sides of the cap layers 4 a to 4 d that the cap layers4 a to 4 d are stepped over. As a result, the gate electrodes of thedouble-gate type can be provided for each both sides of the silicon Fins3 a to 3 d.

As shown in FIG. 14, the conducting film 6 is etched by the RIEtechnology, by using the cap layers 4 a to 4 d and the silicon oxidelayer 2 as a mask and the resist film 7 as a stopper. As a result, thegate electrodes 6 a to 6 j configured by the Poly-Silicon are formed.Until the fabrication process of the gate electrode, all transistors TR1to TR6 in the SRAM Cell can be configured with back-gate type Fin FETs.It is not conducted with each gate electrodes G1 and G2 of alltransistors TR1 to TR6.

Thus, until the fabrication process of the gate electrode, all of thetransistors TR1 to TR6 can be configured with the back-gate type FinFETs of the same structure. Therefore, a plurality of gate electrodes ofthe same structure and a plurality of Fins of the same structure areonly formed during the course of the processing of a plurality of FinFETs. Therefore, enough process margins can be allowed for thelithography processing. If the structures of the gate electrode and theFin of the Fin FETS are various or the Fin FETs more than two varietiesexist, the fabrication processing becomes complex. Moreover, the numberof control parameters in the fabrication processing increases, and thenthe process margins become small and the fabrication processing stepsbecome difficult.

As shown in FIG. 15, the cap layers 4 a to 4 d are etched by the RIEtechnology, by using the silicon Fin 3 a to 3 d and the silicon oxidelayer 2 as a mask and the resist film 7 as a stopper. As a result, sincethe cap material is removed in the area that will become contact holesin the following processing, the silicon layer 3 is exposed.

As shown in FIG. 16, the resist film 7 is removed.

As shown in FIG. 17, the silicon oxide film used for the interlayerinsulator 9 is deposited on the silicon oxide layer 2, the gateelectrodes 6 a to 6 j, and the cap layers 4 a to 4 d by the CVD method.The surface of the interlayer insulator 9 is planarized by the CMPmethod.

As shown in FIG. 18, the resist film 10 is formed on the silicon oxidefilm of the interlayer insulator 9, and the resist film 10 patterned bythe lithography method. The resist film 10 is patterned according to thepattern of the contact holes 8 a to 8 j and 11 a to 11 j describedlater.

As shown in FIG. 19 and FIG. 20, the area of the contact holes 8 a to 8j and 11 a to 11 j of the silicon oxide film of the interlayer insulator9 is etched by the RIE technology, by using the resist film 10 and thecap layers 4 a to 4 d as a mask and the silicon Fins 3 a to 3 d and thegate electrodes 6 a to 6 j as a stopper. As a result, the contact holes8 a to 8 j and 11 a to 11 j are formed.

As shown in FIG. 21, the conducting film used for the contact plug isdeposited by the CVD method, and the conducting film is embedded in thecontact holes 8 a to 8 j and 11 a to 11 j. The conducting film that isdeposited outside of the contact holes 8 a to 8 j and 11 a to 11 j ispolished by the CMP method, by using the silicon oxide film of theinterlayer insulator 9 as a stopper. As a result, the contact plugs 8 ato 8 j and 12 a to 12 j are formed.

As shown in FIG. 22 and FIG. 23, the conducting film used for the M1electrode layer is deposited on the interlayer insulator 9 by thesputtering technique etc. The resist film is formed on the conductingfilm, and the resist film is patterned by the lithography method. Theresist film is patterned according to the pattern of the M1 electrodelayers 13 a to 13 n described later. The conducting film is etched bythe RIE technology, by using the resist film as a mask and theinterlayer insulator 9 as a stopper.

As a result, the M1 electrode layers 13 a to 13 n are formed. And then,the transistors TR1 to TR4 used as the usual (narrowly-defined)double-gate MOSFET can be used by connecting both electrodes of thefirst gate electrode G1 that is the top gate and the second gateelectrode G2 that is the back gate, at the M1 electrode layers 13 a and13 m applying the same voltage.

As for the transfer transistors TR5 and TR6 used as the back-gate typeMOSFET, the gate electrode G1 is coupled to the word line WL, and thegate electrode G2 is coupled to the threshold voltage control line VtC,by the separated M1 electrode layer. Thus, the back-gate type Fin FETand the usual double-gate Fin FET is fabricated to be divided in oneSRAM Cell. That is, all transistors TR1 to TR6 are firstly formed as theback-gate type Fin FET, and then the double-gate Fin FET can beconfigured by connecting the gate electrodes G1 and G2 by the M1electrode layer if necessary. In one SRAM Cell, the back-gate type FinFET and the double-gate Fin FET are consolidated to form the SRAM Cell.

As shown in FIG. 24, the silicon oxide film used for the interlayerinsulator 14 is deposited on the interlayer insulator 9 and the M1electrode layers 13 a to 13 n by the CVD method. The surface of theinterlayer insulator 14 is planarized by the CMP method.

As shown in FIG. 25, the resist film is formed on the interlayerinsulator 14, and the resist film is patterned by the lithographymethod. The resist film is patterned according to the pattern of theVIA1 hole 15 a. The area of the VIA1 plugs 16 a to 16 j of theinterlayer insulator 14 are etched by the RIE technology, by using theresist film as a mask and the M1 electrode layers 13 a to 13 n as astopper. As a result, the VIA1 hole 15 a is formed.

As shown in FIG. 26, the conducting film used for the VIA1 plugs 16 a to16 j is deposited by the CVD method, and the conducting film is embeddedin the VIA1 hole 15 a. The conducting film that is deposited outside ofthe VIA1 hole 15 a is polished by the CMP method, by using theinterlayer insulator 14 as a stopper. As a result, the VIA1 plugs 16 ato 16 j are formed.

As shown in FIG. 27 and FIG. 28, the conducting film used for the wordline WL and the M2 electrode layer is deposited on the interlayerinsulator 14 by the sputtering technique. The resist film is formed onthe conducting film, and the resist film is patterned by the lithographymethod. The resist film is patterned according to the pattern of theword line WL and the M2 electrode layers 17 b to 17 g, 17 l and 17 j.The conducting film is etched through the RIE technology with the resistfilm as a mask and the interlayer insulator 14 as a stopper. As aresult, the pattern of the word line WL and the M2 electrode layers 17 bto 17 g, 17 i and 17 j are formed.

As shown in FIG. 29, the silicon oxide film used for the interlayerinsulator 18 is deposited on the interlayer insulator 14, the word lineWL, and the M2 electrode layers 17 b to 17 g, 17 i and 17 j by the CVDmethod. The surface of the interlayer insulator 18 is planarized by theCMP method. The resist film is formed on the interlayer insulator 18,and the resist film is patterned by the lithography method. The resistfilm is patterned according to the pattern of the VIA2 plugs 19 a to 19h. The area of the VIA2 plugs 19 a to 19 h of the interlayer insulator18 is etched by the RIE technology, by using the resist film as a maskand the word line WL and the M2 electrode layers 17 b to 17 g, 17 i and17 j as a stopper. As a result, the VIA2 hole is formed. The conductingfilm used for the VIA2 plugs 19 a to 19 h is deposited by the CVDmethod, and the conducting film is embedded in the VIA2 hole. Theconducting film that is deposited outside of the VIA2 hole is polishedby the CMP, by using the interlayer insulator 18 as a stopper. As aresult, the VIA2 plugs 19 a to 19 h are formed.

As shown in FIG. 30, the conducting film used for the threshold voltagecontrol line VtC, the bit lines BLT and BLC, the ground lines VSS1 andVSS2 for the ground level, and the power supply voltage VDD is depositedon the silicon oxide film of the interlayer insulator 18 by thesputtering technique. The resist film is formed on the conducting film,and the resist film is patterned by the lithography method. The resistfilm is patterned according to the pattern of the threshold voltagecontrol line VtC, the bit lines BLT and BLC, the ground lines VSS1 andVSS2 for the ground level, and the power supply voltage VDD. Theconducting film is etched by the RTE technology, by using the resistfilm as mask and the interlayer insulator 18 as a stopper. As a result,the threshold voltage control line VtC, the bit lines BLT and BLC, thepower supply electrical potentials VSS1 and VSS2 for the ground level,and the power supply electrical potential VDD are formed. And then, thethreshold voltage control line VtC is formed orthogonal to the word lineWL. And then, compared with the conventional type, the area of the addedpart of the SRAM Cell can be minimized by sharing the threshold voltagecontrol line VtC with the adjacent SRAM Cell in the direction elongatingwith the word line WL.

As shown in FIG. 4 and FIG. 5, the silicon oxide film and thesilicon-nitride film used for the passivation layer 20 are deposited onthe interlayer insulator 18, the threshold voltage control line VtC, thebit lines BLT and BLC, the ground lines VSS1 and VSS2 for the groundlevel, and the power supply voltage VDD by the CVD method. Thefabrication method of the semiconductor memory of the first embodimentis completed by the above-mentioned step.

Although the electrode wiring explained above is fabricated bypatterning the conducting film by the etching method, it is needless tosay, a damascene method can be applied for the fabrication of theelectrode wiring. In the damascene method, the interlayer insulator ispreviously etched to form a trench, and the metal layer is deposited tofill over the trench. And then, the surface of the metal layer isplanarized by the CMP method. As a result, the metal electrode wiring isformed.

Second Embodiment

A semiconductor memory according to the second embodiment of the presentinvention includes a SRAM Cell as shown in FIG. 31. The SRAM Cellincludes six transistors TR1 to TR6 similar to the SRAM Cell shown inFIG. 1 of the first embodiment. However, in the second embodiment, theconnecting destination of the gate electrode G2 of the transistor TR3 toTR6 is different from the first embodiment. In the second embodiment,the gate electrode G2 of the driver transistor TR3 is connected with thethreshold voltage control line VtC. The gate electrode G2 of the drivertransistor TR4 is also connected with the threshold voltage control lineVtC. The gate electrode G2 of the transfer transistor TR5 is connectedwith the word line WL with the gate electrode G1 of the transfertransistor TR5. The gate electrode G2 of the transfer transistor TR6 isconnected with the word line WL with the gate electrode G1 of thetransfer transistor TR6.

In the semiconductor memory according to the second embodiment, it alsoproposes to use the back-gate type, so-called the separated-gate typeFin FETs (TR3, TR4) of the double-gate, as shown in FIG. 31 as a methodof configuring the SRAM Cell by using the Fin FET. In FIG. 31, thethreshold voltage control line VtC is connected with the gate electrodeG2 that is the back-gate of the driver transistor TR3 and is connectedwith the gate electrode G2 that is the back-gate of the drivertransistor TR4. The gate electrodes G1 and G2 of the load transistorTR1, and the gate electrodes G1 and G2 of the load transistor TR2connect with the gate electrode G1 that is a top gate of the drivertransistor TR3, and the gate electrode G1 that is a top gate of thedriver transistor TR4, respectively. Thus, a different voltage isapplied in the gate electrodes G1 and G2 of the driver transistor TR3. Adifferent voltage is also applied in the gate electrodes G1 and G2 ofthe driver transistor TR4.

As shown in FIG. 32, the signal voltage applied to the threshold voltagecontrol line VtC changes synchronizing with the write enable signal WRof the SRAM Cell. The second gate electrodes G2 of the driver transistorTR3 and the driver transistor TR4 are connected with the thresholdvoltage control line VtC. Therefore, the applied voltage for the secondgate electrodes G2 of the driver transistor TR3 and the drivertransistor TR4 changes synchronizing with the write enable signal WR ofthe SRAM Cell, and the value of the threshold voltage of the drivertransistors TR3 and TR4 becomes low synchronizing with write enablesignal WR.

The device characteristic of the driver transistor TR3 and the drivertransistor TR4 can be changed at the time of writing mode and at thetime of data storing mode, by changing the applied signal voltage tothreshold voltage WR.

The back-gate type MOSFET is used for driver transistor TR3 and drivertransistor TR4. The voltage for about half the value of the power supplyvoltage VDD is applied to the threshold voltage control line VtC, andthe voltage for about half the value of the power supply voltage VDD isapplied to the gate electrode G2 of the driver transistor TR3 and thedriver transistor TR4, as the low threshold voltage (low-Vt) mode at thetime of writing the SRAM Cell. As a result, the value of the thresholdvoltage relating to the gate electrode G1 of the driver transistor TR3and the driver transistor TR4 can be reduced, and a large amount ofcurrent can be conducting through the driver transistor TR3 and thedriver transistor TR4.

Furthermore, the more large voltage for about the value of the powersupply voltage VDD is applied to the threshold voltage control line VtC,and the voltage for about the value of the power supply voltage VDD isapplied to the gate electrode G2 of the driver transistor TR3 and thedriver transistor TR4, as the low threshold voltage (low-Vt) mode at thetime of writing the SRAM Cell. It is possible to conduct the largeramount of current through the driver transistor TR3 and the drivertransistor TR4 by realizing the both sides channel that the channel isgenerated on both sides of the gate electrode G1 side and the gateelectrode G2 side.

On the other hand, the voltage for about ground lines voltage controlline VtC, as the high threshold voltage (high-Vt) mode at the time ofstoring data in the SRAM Cell. Therefore, the value of threshold voltagerelating to the gate electrode G1 of the driver transistor TR3 and thedriver transistor TR4 can be increased, the value of the leakage currentcan be reduced, and the SNM can be improved.

In addition, it is necessary to set the value of the voltage for thethreshold voltage control line VtC to the ‘High side’ in large or morethan the value of the potential voltage applied to the word line WL, inthe low threshold voltage (low-Vt) mode. As a result, the value of thethreshold voltage of the driver transistor TR3 and the driver transistorTR4 can be decreased, and the value of the current drive of the drivertransistor TR3 and the driver transistor TR4 can be increased more thanthe value of the current drive of the transfer transistor TR5 and thetransfer transistor TR6.

Thus, the beta ratio of the current drive of the driver transistor tothe current drive of the transfer transistor can be changed by theseries of time variation.

As a result, the SNM can be increased as shown in FIG. 2. That is, ifthe electrical current is conducting with the driver transistor TR3, itbecomes vertical steep in input-output characteristics 31 of the inputvoltage Vin1 and the output voltage Vout1 of the first inverter thatconfigures a flip-flop F/F in the SRAM Cell. Therefore, the SNM can beincreased, since the curve 31 under loop 34 of lower right of thebutterfly curve becomes a convex more below and it obtains the moreoperating margins.

When the electric current is conducting with the driver transistor TR4,the value of the output voltage Vout2 decreases steep in theinput-output characteristics 32 of the input voltage Vin2 and the outputvoltage Vout2 of the second inverter that configures the flip-flop F/F.Therefore, the SNM can be increased since curve 32 under loop 33 ofupper left of the butterfly curve becomes a convex more below and itobtains the more operating margins.

As well as the first embodiment of the present invention, the signalvoltage applied to the threshold voltage control line VtC precedes thetiming of writing/reading the memory cell, and the threshold voltage Vtof the driver transistors TR3 and TR4 is set at earlier time at thetiming of writing/reading the memory cell.

As shown in FIG. 33 and FIG. 34, in the semiconductor memory accordingto the second embodiment of the present invention, silicon Fins 3 a to 3d are provided on a silicon oxide layer 2. It may be similarly to thefirst embodiment of the present invention for the silicon Fins 3 a to 3d.

The silicon Fin 3 a is used for the active area of the driver transistorTR3 and the active area of the transfer transistor TR5. The silicon Fin3 b is used for the active area of the load transistor TR1. The siliconFin 3 c is used for the active area of the load transistor TR2. Thesilicon TR4 and the active area of the transfer transistor TR6.

The cap layers 4 a to 4 d are provided on the silicon Fins 3 a to 3 d,respectively. It may be similarly to the first embodiment of the presentinvention for the cap layers 4 a to 4 d.

The gate electrodes 6 a to 6 j are provided on the silicon oxide layer2. The structure of the gate electrodes 6 a and 6 f is different fromthe structure of the gate electrodes 6 a and 6 f of the first embodimentof the present invention, and the gate electrodes 6 a and 6 f areprovided under the threshold voltage control line VtC. It may besimilarly to the first embodiment for other gate electrodes 6 b to 6 eand 6 g to 6 j.

M1 electrode layers 13 a to 13 n are provided on the interlayerinsulator 9, the contact plugs 8 a to 8 j, and the contact plugs 12 a to12 j.

The M1 electrode layer 13 a connects the gate electrodes 6 b and 6 c. Asa result, the gate electrode G1 of the driver transistor TR3 isconnected with the gate electrodes G1 and G2 of the load transistor TR1.

The M1 electrode layer 13 m connects the gate electrodes 6 g and 6 h. Asa result, the gate electrodes G1 and G2 of the load transistor TR2 areconnected with the gate electrode G1 of the driver transistor TR4.

The M1 electrode layer 13 b connects the gate electrodes 6 d and 6 e. Asa result, the gate electrodes G1 and G2 of the load transistor TR6 areconnected.

The M1 electrode layer 13 k connects the gate electrodes 6 i and 6 j. Asa result, the gate electrodes G1 and G2 of the transfer transistor TR5are connected.

On the other hand, neither gate electrodes 6 a nor 6 b are connected bythe M1 electrode layers. As a result, both of the gate electrodes G1 andG2 of the driver transistor TR3 are not connected, and the drivertransistor TR3 is separated-gate type double-gate FET. Similarly, sinceneither gate electrodes 6 g nor 6 f are connected, neither the gateelectrodes G1 nor G2 of the driver transistor TR4 are connected.Therefore, the driver transistor TR4 is separated-gate type double-gateFET.

The word line WL and the M2 electrode layers 17 b to 17 g, 17 i and 17 jare provided on the interlayer insulator 14 and the VIA1 plugs 16 a to16 j. The word line WL is connected with the gate electrodes 6 d and 6 ethrough the VIA1 plug 16 a, the M1 electrode layer 13 b, and the contactplugs 12 d and 12 e. As a result, the gate electrodes G1 and G2 of thetransfer transistor TR6 are connected with the word line WL.

Moreover, the word line WL is connected with the gate electrodes 6 i and6 j through the VIA1 plug 16 h, the M1 electrode layer 13 k, and thecontact plugs 12 l and 12 j. As a result, the gate electrodes G1 and G2of the transfer transistor TR5 are connected with the word line WL.

The threshold voltage control lines VtC, the bit lines BLT and BLC, theground lines VSS1 and VSS2 for the ground level, and the power supplyvoltage VDD are provided on the interlayer insulator 18 and the VIA2plugs 19 a to 19 h. The threshold voltage control line VtC is connectedwith the gate electrode 6 a through the VIA2 plug 19 a, the M2 electrodelayer 17 e, the VIA1 plug 16 e, the M1 electrode layer 13 c, and thecontact plug 12 a.

As a result, the gate electrode G2 of the driver transistor TR3 isconnected with the threshold voltage control line VtC. Moreover, thethreshold voltage control line VtC is connected with the gate electrode6 f through the VIA2 plug 19 h, the M2 electrode layer 17 f, the VIA1plug 16 f, the M1 electrode layer 13 i, and the contact plug 12 f. As aresult, the gate electrode G2 of the driver transistor TR4 is connectedwith the threshold voltage control line VtC.

In the SRAM Cell, all of the transistors TR1 to TR6 have the first gateelectrode G1 and the second gate electrode G2. As for the transistorsTR1, TR2, TR5 and TR6, the first gate electrode G1 and the second gateelectrode G2 are connected through the M1 electrode layer. As for thedriver transistors TR3 and TR4, the electrode layer is provided for adifferent voltage to be given to the first gate electrode G1 and thesecond gate electrode G2.

In the SRAM Cell of the semiconductor memory according to the secondembodiment, the driver transistors TR3 and TR4 that has back-gate type,so-called separated-gate type structure, and the transistors TR1, TR2,TR5 and TR6 that has common-gate type structure are embedded.

Moreover, until the fabrication process of the gate electrodes G1 andG2, the SRAM Cell can be configured with the double-gate type Fintransistor of the same structure. As a result, a plurality of gateelectrodes of the same structure and a plurality of Fins of the samestructure are only formed during the course of the processing of aplurality of Fin FETs. Therefore, an enough process margins can beallowed for the lithography processing.

If the structure of the gate electrode and the Fin are various, thefabrication processing becomes complex. Moreover, the number of controlparameters in the fabrication processing increases, and then the processmargins become small and the fabrication processing steps becomedifficult.

In the SRAM Cell, the threshold voltage control line VtC that providesthe voltage to the second gate electrode G2 is located orthogonal to theword line WL. The SRAM Cell according to the second embodiment can beconfigured as well as the first embodiment, while following the layoutof the conventional type since the threshold voltage control line VtCintersects with the word line WL. If the threshold voltage control lineVtC is located in parallel to the word line WL, it is necessary toincrease the number of metal layers or it is necessary to bend the wordline WL. Therefore, it is disadvantageous in the processing yield of thesemiconductor memory.

In the SRAM Cell, the threshold voltage control line VtC that providesthe voltage to the second gate electrode G2 of the driver transistorsTR3 and TR4 is shared with an adjacent cell. That is, the semiconductormemory has a plurality of adjacent SRAM Cells mutually sharing thethreshold voltage control line VtC.

The threshold voltage control line VtC that provides the voltage to thesecond gate electrode G2 of the driver transistor TR3/TR4 of a certainSRAM Cell is shared with the threshold voltage control line VtC thatprovides the voltage to the second gate electrode G2 of the drivertransistor TR3/TR4 of an adjacent SRAM Cell in the direction elongatingwith the word line WL.

The minimization of the area can be achieved by sharing the thresholdvoltage control line VtC with an adjacent SRAM Cell. Although the VSSline was shared in the conventional type, two VSS lines are provided inthe cell in the layout of the second embodiment.

(Fabrication Method)

Next, the fabrication method for the semiconductor memory according tothe second embodiment of the present invention will be explained.

Basically, the fabrication method of the semiconductor memory accordingto the second embodiment is similar to the fabrication method of thesemiconductor memory according to the first embodiment. A differentpoint is the shape of the mask for forming the pattern of the siliconFins 6 a to 6 j. Different other respects in the first embodiment andthe second embodiment are: the shape of the mask of the contact layerfor forming the pattern of the contact plugs 12 a to 12; the shape ofmask of the M1 layer for forming the pattern of the M1 electrode layers13 a to 13 n; the shape of mask of the VIA1 layer for forming thepattern of the VIA1 plugs 16 a to 16 j; the shape of mask of the M2layer for forming the pattern of the word line WL and the M2 electrodelayers 17 b to 17 g, 17 i and 17 j; and the shape of the mask of theVIA2 layer for forming the pattern of plugs 19 a to 19 h.

More specifically, the comparison of the semiconductor memory of thesecond embodiment shown in FIG. 35 and the semiconductor memory of thefirst embodiment in FIG. 19 is tried after the contact holes 11 a to 11j are formed, respectively. The shape of the pattern of the silicon Fins6 a and 6 f and the shape of pattern of the contact holes 11 a and 11 fare different in the first embodiment and the second embodiment.

Moreover, comparing of the semiconductor memory of the second embodimentshown in FIG. 36 and the semiconductor memory of the first embodimentshown in FIG. 22 is tried after the M1 electrode layers 13 a to 13 n areformed in each memory. About M1 electrode layer 13 a, although neithergate electrodes 6 a nor 6 b are connected in the second embodiment, thegate electrode 6 a and 6 b are connected in the first of the drivertransistor TR3 are connected in the second embodiment.

About the M1 electrode layer 13 m, although neither gate electrodes 6 gnor 6 f are connected in the second embodiment, the gate electrodes 6 gand 6 f are connected in the first embodiment. As a result, neither gateelectrodes G1 nor G2 of the driver transistor TR4 are connected in thesecond embodiment.

On the other hand, although the gate electrodes 6 i and 6 j of the M1electrode layer 13 k are connected in the second embodiment, neithergate electrodes 6 i nor 6 j are connected in the first embodiment. As aresult, the gate electrodes G1 and G2 of the transfer transistor TR5 areconnected in the second embodiment.

About M1 electrode layer 13 b, although the gate electrodes 6 d and 6 eare connected in the second embodiment, neither gate electrodes 6 d nor6 e are connected in the first embodiment. As a result, the gateelectrode G1 and G2 of the transfer transistor TR6 are connected in thesecond embodiment.

Moreover, the comparison of the semiconductor memory of the secondembodiment shown in FIG. 37 and the semiconductor memory of the firstembodiment shown in FIG. 27 is tried after the word line WL and the M2electrode layers 17 b to 17 g, 17 i and 17 j are formed, respectively.

Although the word line WL is connected with the gate electrodes 6 i and6 j through the VIA1 plug 16 h and the M1 electrode layer 13 k in thesecond embodiment, the word line WL does not connect with gate electrode6 j although the word line WL is connected with the gate electrode 6 iin the first embodiment. As a result, the gate electrodes G1 and G2 ofthe transfer transistor TR5 are connected with the word line WL in thesecond embodiment.

Moreover, although the word line WL is connected with the gateelectrodes 6 d and 6 e through the VIA1 plug 16 a and the M1 electrodelayer 13 b in the second embodiment, the word line WL does not connectwith the gate electrode 6 e although the word line WL is connected withthe gate electrode 6 d in the first embodiment. As a result, the gateelectrodes G1 and G2 of the transfer transistor TR6 are connected withthe word line WL in the second embodiment.

Moreover, comparing of the semiconductor memory according to the secondembodiment shown in FIG. 33 and the semiconductor memory according tothe first embodiment is tried after the threshold voltage control lineVtC, the bit lines BLT and BLC, the ground lines VSS1 and VSS2 at theground level, and the power supply voltage line VDD are formed,respectively.

About the threshold voltage control line VtC, although it connects withthe gate electrode 6 a through the VIA2 plug 19 a, the M2 electrodelayer 17 e, the VIA1 plug 16 e, the M1 electrode layer 13 c, and thecontact plug 12 a in the second embodiment, it connects with the gateelectrode 6 j through the VIA2 plug 19 a, the M2 electrode layer 17 fthe VIA1 plug 16 f, the M1 electrode layer 13 i, and the contact plug 12j in the first embodiment.

Moreover, about the threshold voltage control line VtC, although itconnects with the gate electrode 6 f through the VIA2 plug 19 h, the M2electrode layer 17 f, the VIA1 plug 16 f, the M1 electrode layer 13 i,and the contact plug 12 f in the second embodiment, it connects with thegate electrode 6 e through the VIA2 plug 19 h, the M2 electrode layer 17e, the VIA1 plug 16 e, the M1 electrode layer 13 c, and the electriccontact plug 12 e in the first embodiment.

As well as the first embodiment, until the fabrication process of thegates electrode G1 and G2, all of the transistors TR1 to TR6 can beconfigured with the back-gate Fin FETs of the same structure. Therefore,a plurality of gate electrodes of the same structure and a plurality ofFins of the same structure are only formed during the course of theprocessing of a plurality of Fin FETs. Therefore, enough process marginscan be allowed for the lithography processing.

If the structure of the gate electrode and the Fin are various, thefabrication processing becomes complex. Moreover, the number of controlparameters in the fabrication processing increases, and then the processmargins become small and the improvement of the fabrication processingyield of the semiconductor memory becomes difficult.

And then, the transistors TR1, TR2, TR5 and TR6 used as the usual(narrowly-defined) double-gate MOSFET can be used by connecting bothelectrodes of the first gate electrode G1 that is top gate and thesecond gate electrode G2 that is back gate, at the M1 metal electrodelayers 13 a, 13 m, 13 k and 13 b applying the same voltage.

As for the driver transistors TR3 and TR4 used as the back-gate typeMOSFET, the gate electrode G1 is connected by the silicon Fins 3 b and 3c, and the gate electrode G2 is connected by the threshold voltagecontrol line VtC, by the separated M1 electrode layer.

Thus, the back-gate type Fin FET and usual double-gate Fin FET isfabricated to be divided in one SRAM Cell. That is, all transistors TR1to TR6 are firstly formed as the back-gate type Fin FET, and then thedouble-gate Fin FET can be configured by connecting the gate electrodesG1 and G2 by the M1 electrode layer if necessary. In one SRAM Cell, theback-gate type Fin FET and the double-gate Fin FET are consolidated toform the SRAM Cell in the direction elongating with the word line WL.

And then, the threshold voltage control line VtC is formed orthogonal tothe word line WL. As compared with the conventional type, the area ofthe added part of the SRAM Cell can be minimized by sharing thethreshold voltage control line VtC with the adjacent SRAM Cell.

In the layout of the second embodiment, the threshold voltage controlline VtC is added to the most circumference part of the SRAM Cell aswell as the first embodiment basically, and the threshold voltagecontrol line VtC is shared with the adjacent SRAM Cell.

Moreover, since the electrode wiring of the word line WL is configuredonly by the straight line, the structure of the SRAM Cell becomessimple. Furthermore, all of the transistors TR1 to TR6 can be stillconfigured by the back-gate type Fin FETs. And each top gate isconnected with the M1 electrode layer as for the common-connected gatetype transistors TR1, TR2, TR5 and TR6 that should not be configured bythe back-gate type Fin FETs.

It is configured as the narrowly-defined double-gate Fin FET byconnecting the gate electrode G1 and the gate electrode G2 that isback-gate.

Third Embodiment

A semiconductor memory according to the third embodiment of the presentinvention includes a SRAM Cell as shown in FIG. 38. The SRAM Cellincludes six transistors TR1 to TR6 similar to the SRAM Cell shown inFIG. 31 of the second embodiment. However, in the third embodiment, theconnecting destination of the gate electrode G2 of the transfertransistors TR5 and TR6 is different from the second embodiment.

In the third embodiment, the gate electrode G2 of the transfertransistors TR5 and TR6 are not connected and are made floating states.

In the semiconductor memory according to the third embodiment, it alsoproposes to use the back-gate type, so-called the separated-gate typeFin FETs (TR3, TR4) of the double-gate, as shown in FIG. 38 as a methodof configuring the SRAM Cell by using the Fin FET.

In FIG. 38, the threshold voltage control lines VtC is connected withthe gate electrode G2 that is the back-gate of the driver transistor TR3and is connected with the gate electrode G2 that is the back-gate of thedriver transistor TR4.

Respectively, the gate electrodes G1 and G2 of the load transistor TR1,and the gate electrodes G1 and G2 of the load transistor TR2 connectwith the gate electrode G1 that is a top gate of the driver transistorTR3, and the gate electrode G1 that is a top gate of the drivertransistor TR4.

Thus, a different voltage is applied in the gate electrodes G1 and G2 ofthe driver transistor TR3. A different voltage is also applied in thegate electrodes G1 and G2 of the driver transistor TR4.

In the third embodiment, the signal voltage applied to the thresholdvoltage control line VtC also changes synchronizing with the writeenable signal WR of the SRAM Cell as well as the second embodiment shownin FIG. 32. The device characteristic of the driver transistor TR3 andthe driver transistor TR4 can be changed at the time of writing mode andat the time of data storing mode, by changing the applied signal voltageto threshold voltage control line VtC synchronizing with the writeenable signal WR.

Furthermore, although the gate electrode G2 of the transfer transistorsTR5 and TR6 is connected with the word line WL in the second embodiment,the gate electrode G2 of the transfer transistors TR5 and TR6 is madefloating in the third embodiment.

As a result, in the third embodiment, the current drive of the transfertransistor can be decreased as compared with the second embodiment. Andthen, in the third embodiment, the beta ratio of the current drive ofthe driver transistor to the current drive of the transfer transistorcan be increased more than the value of the beta ratio in the secondembodiment. As a result, the SNM can be increased.

As for the semiconductor memory according to the third embodiment shownin FIG. 38, the silicon Fins 3 a to 3 d are provided on the siliconoxide layer 2 as shown in FIG. 39 and FIG. 40. It may be similar to thefirst embodiment for the silicon Fins 3 a to 3 d and the gate electrodes6 a to 6 j. It only has to make the gate electrode 6 d floating withoutany connecting.

Alternatively, even if the contact plug 12 d is connected with the gateelectrode 6 d, it only has to make the gate electrode 6 d and contactplug 12 d floating with another.

Moreover, the gate electrode 6 i is also made floating without anyconnecting. Alternatively, even if the electric contact plug 12 i isconnected with the gate electrode 6 i, it only has to make the gateelectrode 6 i and the electric contact plug 12 i floating with another.

As well as the first embodiment and the second embodiment, until thefabrication process of the gate electrode, all of the transistors TR1 toTR6 can be configured by the back-gate Fin FETs of the same structure.As a result, a plurality of gate electrodes of the same structure and aplurality of Fins of the same structure are only formed during thecourse of the processing of a plurality of Fin FETs. Therefore, enoughprocess margins can be allowed for the lithography processing.

Other Embodiments

As described above, the present invention is described according to thefirst through the third embodiment; however, it should not be perceivedthat descriptions and drawings forming a part of this disclosure areintended to limit the spirit and scope of the present invention. Variousalternative embodiments, working examples, and operational techniqueswill become apparent from this disclosure for those skills in the art.

For example, all the double-gate type transistor and back-gate typetransistors in the SRAM Cell are not limited to the Fin FETs, and thepart of the six transistors configuring the SRAM Cell can be realized bythe planar-type double-gate MOSFETs.

The basic circuit operation of the SRAM Cell can be realized and a largeamount of SNM can be achieved by using the double-gate MOSFETs for thetransistor layout pattern and the circuit representation of the SRAMCell similar to the first embodiment to the third embodiment.

Moreover, the layout pattern of the SRAM Cell of the semiconductormemory of the present invention is not limited to the layout pattern ofthe first embodiment to the third embodiment. Being possible toconfigure is needless to say even if another layout pattern is used.

As such, the present invention naturally includes various embodimentsnot described herein. Accordingly, the technical scope of the presentinvention is determined only by specified features of the inventionaccording to the following claims that can be regarded appropriate fromthe above-mentioned descriptions.

1. A semiconductor memory having a plurality of static random accessmemory cells, a plurality of word lines, and a plurality of first andsecond bit lines substantially orthogonal to the word lines, each of thestatic random access memory cell comprising: a first inverter having afirst driver transistor and a first load transistor connected in seriesbetween a power supply voltage line and a ground line; a second inverterhaving a second driver transistor and a second load transistor connectedin series between the power supply voltage line and a ground line; afirst transfer transistor connected in series between a first bit lineand an output of the first inverter; and a second transfer transistorconnected in series between a second bit line and an output of thesecond inverter, the output of the first inverter being connected to aninput of the second inverter and an input of the first inverter beingconnected to the output of the second inverter, wherein at least one ofthe first and the second driver transistors, the first and the secondload transistors, and the first and the second transfer transistors isconfigured by a plurality of Fin field effect transistor, and the Finfield effect transistor is configured by a separated-gate typedouble-gate field effect transistor comprising a first gate electrodeand a second gate electrode, controlling a voltage for the first gateelectrode to form a channel, and controlling a voltage for the secondgate electrode to decrease a threshold voltage at the time of writingdata.
 2. The semiconductor memory of claim 1, wherein the appliedvoltage for the second gate electrode changes synchronizing with a writeenable signal of the static random access memory cell, and the thresholdvoltage of the double-gate field effect transistor lowers synchronizingwith the write enable signal.
 3. The semiconductor memory of claim 2,wherein the applied voltage for the second gate electrode synchronizingwith the write enable signal changes from a time earlier than a timingof rising of the write enable signal, and changes until a time laterthan a timing of falling of the write enable signal.
 4. A semiconductormemory having a plurality of static random access memory cells, aplurality of word lines, and a plurality of first and second bit linessubstantially orthogonal to the word lines, each of the static randomaccess memory cell comprising: a first inverter having a first drivertransistor and a first load transistor connected in series between apower supply voltage line and a ground line; a second inverter having asecond driver transistor and a second load transistor connected inseries between the power supply voltage line and a ground line; a firsttransfer transistor connected in series between a first bit line and anoutput of the first inverter; and a second transfer transistor connectedin series between a second bit line and an output of the secondinverter, the output of the first inverter being connected to an inputof the second inverter and an input of the first inverter beingconnected to the output of the second inverter, wherein the first andthe second driver transistors, the first and the second loadtransistors, and the first and the second transfer transistors areconfigured by a plurality of Fin field effect transistors, and both thefirst transfer transistor and the second transfer transistor areconfigured by a separated-gate type double-gate field effect transistorcomprising a first gate electrode and a second gate electrode,controlling a voltage for the first gate electrode to form a channel,and controlling a voltage for the second gate electrode to decrease athreshold voltage at the time of writing data.
 5. The semiconductormemory of claim 4, wherein the applied voltage for the second gateelectrode changes synchronizing with a write enable signal of the staticrandom access memory cell, and the threshold voltage of the double-gatefield effect transistor lowers synchronizing with the write enablesignal.
 6. The semiconductor memory of claim 5, wherein the appliedvoltage for the second gate electrode synchronizing with the writeenable signal changes from a time earlier than a timing of rising of thewrite enable signal, and changes until a time later than a timing offalling of the write enable signal.
 7. The semiconductor memory of claim4, wherein the first load transistor is configured to have a first gateelectrode and a second gate electrode commonly connected to the input ofthe first inverter; and the second load transistor is configured to havea first gate electrode and a second gate electrode commonly connected tothe input of the second inverter.
 8. The semiconductor memory of claim4, wherein the first driver transistor is configured to have a firstgate electrode and a second gate electrode commonly connected to theinput of the first inverter; and the second driver transistor isconfigured to have a first gate electrode and a second gate electrodecommonly connected to the input of the second inverter.
 9. Thesemiconductor memory of claim 4, wherein the first gate electrodes ofthe first transfer transistor and the second transfer transistor areconnected to the word line, respectively; and the second gate electrodesof the first transfer transistor and the second transfer transistor areconnected to threshold voltage control lines, respectively.
 10. Thesemiconductor memory of claim 9, wherein the threshold voltage controllines are located substantially orthogonal to the word line.
 11. Thesemiconductor memory of claim 10, wherein the threshold voltage controllines are mutually shared with static random access memory cellsadjacently disposed in the direction elongating with the word lines. 12.A semiconductor memory having a plurality of static random access memorycells, a plurality of word lines, and a plurality of first and secondbit lines substantially orthogonal to the word lines, each of the staticrandom access memory cell comprising: a first inverter having a firstdriver transistor and a first load transistor connected in seriesbetween a power supply voltage line and a ground line; a second inverterhaving a second driver transistor and a second load transistor connectedin series between the power supply voltage line and a ground line; afirst transfer transistor connected in series between a first bit lineand an output of the first inverter; and a second transfer transistorconnected in series between a second bit line and an output of thesecond inverter, the output of the first inverter being connected to aninput of the second inverter and an input of the first inverter beingconnected to the output of the second inverter, wherein the first andthe second driver transistors, the first and the second loadtransistors, and the first and the second transfer transistors areconfigured by a plurality of Fin field effect transistors, and both thefirst driver transistor and the second driver transistor are configuredby a separated-gate type double-gate field effect transistor comprisinga first gate electrode and a second gate electrode, controlling avoltage for the first gate electrode to form a channel, and controllinga voltage for the second gate electrode to decrease a threshold voltageat the time of writing data.
 13. The semiconductor memory of claim 12,wherein the voltage for the second gate electrode changes synchronizingwith a write enable signal of the static random access memory cell, andthe threshold voltage of the double-gate field effect transistor lowerssynchronizing with the write enable signal.
 14. The semiconductor memoryof claim 13, wherein the voltage for the second gate electrodesynchronizing with the write enable signal changes from a time earlierthan a timing of rising of the write enable signal, and changes until atime later than a timing of falling of the write enable signal.
 15. Thesemiconductor memory of claim 12, wherein the first load transistor isconfigured to have a first gate electrode and a second gate electrodecommonly connected to the input of the first inverter; and the secondload transistor is configured to have a first gate electrode and asecond gate electrode commonly connected to the input of the secondinverter.
 16. The semiconductor memory of claim 12, wherein the firsttransfer transistor is configured to have a first gate electrode and asecond gate electrode commonly connected to the word line; and thesecond transfer transistor is configured to have a first gate electrodeand a second gate electrode commonly connected to the word line.
 17. Thesemiconductor memory of claim 12, wherein both the first transfertransistor and the second transfer transistor are configured by aseparated-gate type double-gate field effect transistor comprising afirst gate electrode and a second gate electrode, controlling a voltagefor the first gate electrode to form a channel, and making the secondgate electrode electrically floating.
 18. The semiconductor memory ofclaim 12, wherein the first gate electrode of the first drivertransistor is connected to the input of the first inverter and thesecond gate electrode of the first driver transistor is connected to afirst threshold voltage control line; and the first gate electrode ofthe second driver transistor is connected to the input of the secondinverter and the second gate electrode of the second driver transistoris connected to a second threshold voltage control line.
 19. Thesemiconductor memory of claim 18, wherein the first and the secondthreshold voltage control lines are located substantially orthogonal tothe word line.
 20. The semiconductor memory of claim 19, wherein thefirst and the second threshold voltage control lines are usually sharedwith static random access memory cells adjacently disposed in thedirection elongating with the word line.